Generation of a dc voltage by an ac magnetic field in type-II superconductors

We show that an ac external magnetic field can generate a dc voltage in type-II superconductors carrying a constant transport current. This rectifying effect occurs even at low temperatures where flux creep may be disregarded and even for arbitrarily small applied current. The dc signal appears when the magnitude of the applied ac field exceeds a threshold value which depends on the shape of the superconductor and on the applied current. Experiments on this subject are discussed.